CIC 2055 When someone asks him, “Which commandment in the Law is the greatest?”1 Jesus replies: “You shall love the Lord your God with all your heart, and with all your soul, and with all your mind. This is the greatest and first commandment. And a second is like it: You shall love your neighbor as yourself. On these two commandments hang all the Law and the prophets.”2 The Decalogue must be interpreted in light of this twofold yet single commandment of love, the fullness of the Law:
The commandments: “You shall not commit adultery, You shall not kill, You shall not steal, You shall not covet,” and any other commandment, are summed up in this sentence: “You shall love your neighbor as yourself.” Love does no wrong to a neighbor; therefore love is the fulfilling of the law.3

CIC 2196 In response to the question about the first of the commandments, Jesus says: “The first is, ‘Hear, O Israel: The Lord our God, the Lord is one; and you shall love the Lord your God with all your heart, and with all your soul, and with all your mind, and with all your strength.’ The second is this, ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”4
The apostle St. Paul reminds us of this: “He who loves his neighbor has fulfilled the law. The commandments, ‘You shall not commit adultery, You shall not kill, You shall not steal, You shall not covet,‘ and any other commandment, are summed up in this sentence, ‘You shall love your neighbor as yourself.’ Love does no wrong to a neighbor; therefore love is the fulfilling of the law.”5

CIC 2811 In spite of the holy Law that again and again their Holy God gives them – “You shall be holy, for I the LORD your God am holy” – and although the Lord shows patience for the sake of his name, the people turn away from the Holy One of Israel and profane his name among the nations.6 For this reason the just ones of the old covenant, the poor survivors returned from exile, and the prophets burned with passion for the name.

1 Mt 22:36.
2 Mt 22:37-40; cf. Deut 6:5; Lev 19:18.
3 Rom 13:9-10.
4 Mk 12:29-31; cf. Deut 6:4-5; Lev 19:18; Mt 22:34-40; Lk 10:25-28.
5 Rom 13:8-10.
6 Ezek 20:9, 14, 22, 39; cf. Lev 19:2.